1. Field of the Invention
This invention relates to smoke detectors. The invention relates more particularly to an improved smoke detector of the ionization type.
2. Description of the Prior Art
Ionization type smoke detectors are known and have been used as combustion product detectors in home and industrial applications for early warning of fire. The ionization detector includes, generally, an ionization chamber having first and second electrodes, a means for establishing an electric field between these electrodes, and means for causing ionization within the chamber. In one form of detector, ionization is produced by exposing air particles to a radioactive source located within the chamber. Charged particles comprising ions are produced by radiation and an ion current flows between the electrodes.
Detection of airborne products of combustion is provided when these products enter the ionization chamber. A chamber inlet provides access for entry of these products into the chamber. In operation, the charged particles produced by radiation within the chamber establish a quiescent ion current. As combustion products enter the chamber, the charged particles attach themselves to these products and the magnitude of the ion current is reduced. The reduction in ion current amplitude is sensed by circuit means and when the current is reduced to a predetermined level, an electrical signal is generated which initiates a visible or audible warning indication.
Heretofore, the design of ionization smoke detectors has, for practical considerations, required a compromise in certain important characteristics of the detector. One such characteristic is the sensitivity of the detector to the presence of combustion products. A relatively high sensitivity is desirable since it will provide an early warning of the presence of airborne combustion products and provide relatively more time for reacting to a potentially dangerous condition. On the otherhand, ionization detectors have exhibited an instability attributable to air currents which operate to trigger false alarms. Air currents flowing through the ionization chamber carry some of the ions from the chamber and cause a reduction in quiescent ion current which triggers a false alarm. In order to reduce the effect of this instability, the sensitivity of the detector to smoke has been limited by establishing a relatively large, predetermined reduction in the amplitude of ion current before a smoke detection signal is generated. In addition, although it is desirable to provide optimum inlet dimensions to the chamber in order to enhance the capture of airborne products of combustion, the instability caused by air currents has necessitated the use of relatively small or well baffled inlets which limit access to the chamber of airborne combustion products. The sensitivity of the detector to airborne combustion products has thus been reduced in order to enhance the stability of the detector and to avoid the generation of false alarms. It would be desirable to provide an ionization detector wherein the susceptibility of the detector to false alarms caused by air currents is substantially reduced.
It is known that the sensitivity of ionization smoke detectors can be enhanced by establishing a unipolar region within the ionization chamber. The ion current produced by a radioactive source comprises charged particles of opposite polarities. A number of these particles will recombine and reduce the quiescent amplitude of charged particles available for attachment to airborne combustion products. A decrease in the magnitude of ion current in the presence of combustion products is consequently not as pronounced as is desirable and the sensitivity of the detector to those products is accordingly reduced. The provision of a unipolar region within the chamber enhances sensitivity. Charged particles within a unipolar region are of the same polarity and the probability of recombinations within this region is reduced. While prior ionization smoke detection arrangements are known which have provided means for establishing a unipolar region, these arrangements have been relatively complex or relatively costly or both.